The present invention relates to a method and apparatus for generating dither patterns for use in converting a gray-scale image to a bi-level image having the appearance of an image photographed through a screen.
Dithering converts a monochrome or color gray-scale image to a monochrome or color bi-level image in which different gray levels are expressed by different densities of equal-sized dots, so that the image can be printed or displayed by a device capable only of bi-level image output. Dithering employs a matrix of threshold values with which the values of picture elements or pixels in the image are compared, the same matrix pattern being employed repeatedly to cover the image.
Photographing an image through a screen is a technique used to produce the halftone images printed in newspapers and magazines. This technique produces a bi-level image comprising a rectilinear pattern of dots, in which different gray levels are expressed by different dot sizes. To reduce the obviousness of the dot pattern, the screen is generally tilted at an angle, referred to as the screen angle. Halftone printing can reproduce images with a quite natural appearance, so there is interest in simulating this appearance by dithering.
Creating a dither matrix that simulates a halftone screen with a screen angle of forty-five degrees is not difficult, but for arbitrary screen angles, matters are not simple. Rotating a standard pattern of threshold values through the desired screen angle creates problems in distributing the threshold values into the dither matrix cells, and in aligning adjacent copies of the dither matrix to cover the image. Furthermore, if the dither matrix has the wrong size, extraneous dots that interfere with the halftone effect may be produced in the output bi-level image, as will be illustrated later.